A centrifugal compressor employed in a gas turbine or the like typically includes an impeller including a plurality of rotor blades arranged circumferentially at a prescribed interval, a shroud provided so as to oppose the impeller to define an annular impeller inlet portion opening in an axial direction and an annular outlet portion opening radially outward in cooperation with the impeller, a diffuser having an annular diffuser inlet portion opposing the impeller outlet portion, and a row of stator vanes arranged circumferentially in the diffuser inlet portion at a prescribed interval. See JP2001-304186A and JP2001-342995, for instance. Such a centrifugal compressor may be referred to as closed impeller centrifugal compressor.
In a known diffuser for a centrifugal compressor provided with a row of stator vanes, a plurality of openings are provided in an inlet part of the diffuser so as to correspond to the stator vanes, and these openings are communicated with a common annular passage. See JPH10-37899A.
In an impeller outlet portion of a centrifugal compressor, a high pressure zone is created in a front face part of each rotor blade, and a low pressure zone is created in a rear face part of each rotor blade. The working fluid such as air flowing from the impeller outlet portion to the diffuser inlet portion preserves this pressure distribution into the diffuser inlet portion.
As the working fluid is forwarded by the impeller from the impeller outlet portion to the diffuser inlet portion, a certain pressure distribution is created along the leading edges of the stator vanes. The rotor blades of the impeller pass the stator vanes of the diffuser as the impeller rotates, and this causes a periodic disturbance in the pressure distribution in the impeller outlet portion. The resulting periodic change in the pressure distribution in the impeller outlet portion causes vibration of the impeller blades. Such a vibration is undesirable for the durability of the impeller blades, and may even cause a destructive contact between the impeller blades and the shroud surrounding the impeller in extreme cases.
The prior art disclosed in JPH10-37899A is intended as a solution to the problem of the pressure fluctuation in the impeller outlet portion and the diffuser inlet portion in the hope of equalizing the pressure in the circumferential direction via the small openings and the annular passage provided in the diffuser inlet portion. However, since the pressure distribution is cyclic along the circumferential direction, an effective equalization of pressure cannot be expected if the small openings are provided at the same pitch as the impeller blades. Therefore, this prior art is not likely to be effective in preventing the vibration of the impeller blades, and increasing the durability of the impeller blades.